There are already known to exist various wavelength demultiplexing techniques, especially those techniques using thin layer filters, namely those which use directional waveguide couplers and those which use diffraction grids. However, the techniques mentioned above are unsuitable for the low cost demultiplexing of multiplexed wavelength optical signals whose respective wavelengths are very close to each other and, for example, are separated by merely a few tenths of one nanometer.
In effect, the thin layer filters and the directional waveguide couplers have wide optical passbands of, for example, about 70 nanometers and in addition are not wavelength-tuneable. The diffraction grids, which allowed for the above-envisaged demultiplexing (slightly spaced wavelengths), can theoretically be embodied, but would involve demultiplexers whose dimensions and cost is prohibitive, given the fact that a commerical diffraction grid demultiplexer already has a length of 8 cm for a wavelength selectivity of merely 0.163 nm (that is, it only makes it possible to separate two optical signals if the wavelengths of the latter are distant by more than 0.163 nm).
It is also possible to consider using for the envisaged demultiplexing a technique using periodical filters or even a technique using an independent heterodyne detection. However, such technques have the drawback of requiring--as have all the techniques also mentioned previously--the use of rapid photodetectors so as to convert the demultiplexed optical signals into electric signals.
Finally and in particular via the article by H. KAWAGUCHI et al published in Applied Physics Letters 50(2) Jan. 12th 1987, p. 66 and 67 and via the article by H. NAKAJIMA published in the reports of the ECOC 87 conference, p. 121 to 124 to which said articles refer, there is known to exist a technique suitable for the demultiplexing envisaged above and which to this effect uses a resonant type semiconductor laser amplifier (SLA). This amplifier may a Fabry-Perot or distributed feedback (DFB) type amplifier. Such devices are wavelength-tuneable optical filters which have a high optical gain and are thus able to be used both as amplifiers and optical filters.
However, a resonant type semiconductor laser amplifier also needs to be connected to a rapid photodetector so as to convert the optical signal it supplies into an electric signal.